A defibrillator is a therapeutic instrument that is used to assist in curing certain cardiac irregularities, particularly ventricular and atrial fibrillation. The defibrillator is typically interconnected with a patient via a pair of paddle electrodes and associated conductors, and functions to apply a high energy pulse to the patient via the electrodes when appropriately triggered by an operator. A defibrillator is frequently used in conjunction with an ECG monitor that provides a display of a patient's ECG waveform and that may also be interconnected with the patient via a set of electrodes. In one type of prior physiological instrument, a combined defibrillator/ECG monitor is provided in which the electrodes normally used for defibrillation can be used to provide the ECG signal to the ECG monitor.
One common method of positioning defibrillator electrodes on a patient is referred to as the anterior/anterior technique. In this technique, one electrode is placed lateral to the upper sternum and below the right clavicle on the patient's right chest (the "sternum" position), and the second electrode is placed on the patient's lower left chest, usually just below and lateral to the cardiac apex (the "apex" position). The terms "right" and "left" are here used in relation to the patient. Since the operator will be facing the patient during defibrillation, the electrode in the sternum position will be held in the operator's left hand, and the electrode in the apex position will be held in the operator's right hand.
Defibrillation efficiency is generally considered to be unaffected by the polarity of the sternum and apex electrodes, i.e., an effective defibrillation shock can be applied regardless of whether the electrode in the sternum position is negative or positive with respect to the electrode in the apex position. Nevertheless, it is a longstanding convention in the health care field to apply a defibrillation pulse with the sternum electrode predominantly negative with respect to the apex electrode. The importance of this convention lies principally in the fact that the electrodes are often used as the source of an ECG signal. When the defibrillator electrodes are used to provide the ECG signal, use of the electrodes in preestablished apex and sternum positions ensures that the resulting ECG display or printout will not be inverted with respect to its normal form. An inverted ECG signal could conceivably be misinterpreted, particularly under emergency conditions. Most prior defibrillators have therefore included electrodes labeled with the terms "Sternum" and "Apex" to facilitate proper placement of the electrodes on the patient. However, labeling the electrodes does not guarantee that an operator will place them in their proper positions.